As is known, many pourable food products, such as fruit juice, UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as TETRA BRIK Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or mineral-filled polypropylene material; and a number of lamination layers of heat-seal plastic material, e.g. polyethylene films, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
Packages of this sort are normally produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating; and the web of packaging material so sterilized is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is filled with the sterilized or sterile-processed food product, and is sealed and subsequently cut along equally spaced cross sections to form pillow packs, which are then folded mechanically to form respective finished, e.g. substantially parallelepiped-shaped, packages.
Alternatively, the packaging material may be cut into blanks, which are formed into packages on forming spindles, and the packages are filled with the food product and sealed. One example of this type of package is the so-called “gable-top” package known by the trade name TETRA REX (registered trademark).
To open the packages described above, various solutions have been proposed, including reclosable opening devices made of plastic material and substantially comprising a spout, e.g. tubular, defining a through opening and fitted to a hole in a wall of the package; and a removable, e.g. screw or hinged, cap fitted to and outwardly closing the spout.
When producing the opening device, the opening of the spout is sealed by a plastic membrane connected integrally to the spout and detachable from it along a normally circular tear line; and, on the side facing the cap, the membrane has an integral projecting pull ring, the free end of which is pulled by the user to detach the membrane from the spout along the tear line and so open the product pour opening. More specifically, the pull ring extends inside, and at a predetermined distance from, the spout.
Using the opening devices described, the package is easy to open, with a reasonable amount of twist required of the user, and the membrane is detached neatly from the spout.
The same does not apply, however, in the case of packages requiring not only liquid but also gas sealing where the opening device is fitted to the packaging material.
In such cases, solutions are known in which the membrane of the opening device is fixed directly over a prelaminated hole in the packaging material, i.e. a hole formed in the base layer only and covered by the other lamination layers, including the layer of gas-barrier material.
Using this solution, opening the package calls not only for detaching the membrane from the spout, but also for tearing the layer of barrier material against the edge of the hole through the base layer. With commonly used barrier materials, such as aluminium, this invariably results in a jagged, frayed edge of the hole, thus impairing smooth pour-out of the food product.